Unidirectional microphone



Dec. 24, 1963 A. M. WIGGINS 3,115,207

UNIDIRECTIONAL MICROPHONE Filed Jan. 11, 1954 3 Sheets-Sheet 1 6/ 57/04 gig w Dec. 24, 1963 A. M. WIGGINS v3, 7

UNIDIRECTIONAL MICROPHONE Filed Jan. 11, 1954 3 Sheets-Sheet 2 United States Patent 3,115,207 UNIDIREETIGNAIJ MIQRGPHONE Alpha Ml. Wiggins, Buchanan, Mich, assignor to Electro- Voice, Incorporated, Buchanan, Mich. Filed Jan. 11, 1954, Ser. No. 4%,W9 18 Claims. (Cl. I8I3i) The present invention relates to a microphone, and more particularly to a unidirectional microphone having a substantially constant response for all frequencies Within a predetermined range.

Heretofore, it has been found advantageous to employ pressure gradient microphones because it was convenient to control their response pattern so as to make them directional. Since the sound responsive element in a pressure gradient microphone is open to the sound medium on both sides, the response is a function of the difference in sound pressure between the two sides of the diaphragm. For a remote sound source, the magnitudes of the sound pressures on the two sides of the diaphragm are substantially the same, although there is a difference in phase between the two sound pressures. The force available for actuating the diaphragm is the vector sum of the two forces on opposite sides. Where the distance between opposite sides of the diaphragm is small compared to the wave length of the sound, the vector sum of the force acting on the diaphragm is proportional to the frequency. Hence, the mechanical impedance of the microphone must be proportional to frequency, if the velocity is to be independent of the frequency. This, therefore, requires the sound responsive diaphragm and element to be mass controlled throughout the pass band to obtain a uniform response.

In order to obtain a uniform response characteristic, some microphones have employed corrugated aluminum ribbons approximately .0001 inch thick, and others have used very flexible diaphragrns, all of which are quite fragile. It is possible to tear out an aluminum ribbon by breathing on the microphone or by swinging the microphone through the air on a boom. A mass controlled microphone is quite susceptible to shock which will produce a highly objectionable voltage output. Such voltage outputs are frequently generated when the microphone is moved on a boom during a television program or during a motion picture sound recording. The force available for actuating the diaphragm of a microphone of this type is proportional to the frequency if the distance D from front to back of the diaphragm is very small compared to the wave length of the sound. Where a pressure gradient microphone has a distance D, which is small, the ratio of signal to shock susceptibility is rather low.

Pressure gradient microphones also have a proximity effect which produces an increase in output at the low frequencies when the source of sound is at a small distance from the microphone. The proximity effect is increased as the distance from the front to the back of the diaphragm is decreased. Therefore, objects of the present invention are to provide a unidirectional pressure gradient microphone which has virtually a constant force on the diaphragm at all frequencies, and which has a negligible proximity effect and also a high signal to shock susceptibility ratio.

Other and further objects of the invention subsequently will become apparent by reference to the following d scription taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a sectional view of a microphone constructed in accordance with the principles of the present invention and somewhat diagrammatically illustrated;

FIGURE 2 is an electrical circuit diagram which is the equivalent of the acoustical circuit embodied in the microphone of FIGURE 1;

FIGURES 3, 4, 5, 6, 7, 8 and 9 are vector diagrams explanatory of the operation of the microphone illustrated in FIGURES 1 and 2;

FIGURE 10 is a front view of a microphone constructed in accordance with the principles of the present invention wherein certain portions have been broken away to illustrate the construction thereof; and

FIGURE 11 is a side view of the microphone shown in FIGURE 10 with portions broken away to further illustrate other constructional details thereof.

If it were possible to vary the distance between the front and back of a diaphragm in a pressure gradient microphone at a rate inversely proportional to the frequency, the vector sum of the pressure on the diaphragm could be made independent of the frequency. Thus, the available force for actuating the diaphragm would be independent of the frequency, and under such conditions the sound responsive element should no longer be mass controlled, but should he resistance controlled. Thus, the sound responsive element could be constructed in the manner of a conventional pressure microphone.

In a wide range dynamic pressure microphone, the diaphragm and voice coil assembly usually resonates between 4-00 and 600 cycles per second. The mechanical system of the diaphragm and voice coil is highly damped so that the stiffness and the mass reactance are small compared with the resistance of the system. With a constant sound pressure available for actuating the diaphragm versus frequency, the velocity of the moving element of such a pressure microphone is independent of the frequency. A constant force on the diaphragm versus frequency can be obtained in a pressure gradient microphone if the effective distance between the sound entrances to opposite sides of the moving element is inversely proportional to the frequency.

In accordance with the present invention, this is accomplished by providing multiple sound entrances or openings to the back of the diaphragm. At least one opening which defines a small distance from the front to the back of the diaphragm gives preference to the transmission of high frequency sound. Another entrance, which defines a relatively long distance from the front to the back of the diaphragm, includes a low pass filter and gives preference to low frequency sound. At intermediate frequencies the sound pressures enter both openings, and the pressures entering from the two openings add vectorially at the back surface of the diaphragm. By selecting the parameters of the low pass filter which is associated with the sound entrance at the greater distance from the diaphragm, the magnitude of a vector sum can be obtained over the pass band of frequencies which will be the equivalent of having a sound opening which moves along the axis of the microphone at a rate inversely proportional to the frequency of the sound.

A structure which accomplishes the foregoing objective is illustrated in FIGURE 1. A microphone is provided which has an electroacoustical transducer with a diaphragm l2 suitably supported by a ring 13 which forms a sound port in a housing 16. The diaphragm 12 carries a voice coil 14 positioned within a suitable annular gap 15 in a magnetic circuit means having an electromagnetic or permanent magnet structure. The gap 15 containing the voice coil 14 is in a supporting structure or housing 16 having a high frequency entrance or opening 17 connected by a passage 13 to the underside of the diaphragm 12. The diaphragm 12 together with the housing 16 defines a chamber I9 which comprises an acoustical capacitance which is designated C in FIGURE 2. The opening 17, passage I8 and chamber 19 form a sound path to the side of the diaphragm opposite the port. The

passage 18 contains a resistance element 21 designated R in FIGURE 2, which is a piece of felt or similar material. For purposes of symmetry, two high frequency openings 1'7 and passages 18 are provided to the chamber 19, thus forming a sound path equal in length to the one described above. The housing also defines a low frequency sound path or transmission line to the chamber 19 comprising a tube 22, which defines a sound entrance or opening 22A for low frequencies at one end and a passage 23. The tube 22, passage 23 and opening 22A form a second sound path to the diaphragm. The passage 23 adjacent its junction with the chamber 19 likewise contains aresistance element 24 such as felt. At suitable intervals along the tube 22 other resistance elements 25 are placed. The tube 22, passage 23 and resistance elements 25' constitute a means for limiting the frequencies transmitted through the opening 22A to a low frequency band.

The housing 16 also contains another chamber 25 connected by a passage 27 to the chamber 19 which has therein a resistance element 28. The resistance element 28, the passage 27, and the chamber 26 constitute a terminating impedance in the form of an acoustical resistance and capacitance for the low frequency transmission line comprising the passages 22 and 23 and their resistance elements 24 and 25. The passages 18, 22, 23 and 27 are all imperforate passages with openings, designated 29, at each end.

The reactance of the inertance of the opening of the passage 23 is low for allowing low frequency sound pressure easy entrance to the chamber 19. The resistances offered by the members 21 in the passages 13 are too high to produce any unidirectional action at low frequencies since the passages are coupled to the large chamber 26, and hence the microphone would tend to be omnidirectional at low frequencies. This omnidirectional characteristic at low frequencies is eliminated by the opening in the end of the chamber 22 at a relatively long distance from the diaphragm 12 and the provision of a terminating impedance for the low frequency transmission line formed by the chamber 22 and passage 23 in the form of the acoustical resistance and capacitance included in the chamber 26, passage 27, and the resistance element 28. By having these properly proportioned, unidirectivity of the microphone is accomplished for low frequencies.

FIGURE 2 is an electrical circuit diagram which is the equivalent of the acoustical system diagrammatically illustrated in FIGURE 1. F is the force generated by the sound impinged upon the outer surface of the diaphragm 12. The inductor and capacitor M and C are the mass and compliance of the diaphragm 12 and the voice coil 14. The capacitance C is the acoustical capacitance of the chamber 19. R is the resistance over the high frequency openings produced by the felt 21, and F is the force due to sound pressure at the high frequency openings or passages 18, which is impressed through the resistance elements R or felt members 21 upon the capacitance C of the chamber 19. The mass reactance of these passages is so small that it is not shown on the diagram. The resistance R is for the two high frequency passages in parallel. M R and C are the inertance, resistance and capacitance of the closed chamber 26, the passage 27 and the resistive element 28. R M are the resistance, and inertance of the low frequency opening 23 and the resistive element 24. M R C and C represent the transmission line lumped constants of the passage 22 and the resistance elements 25, across which is impressed the low frequency force F It is thus clear that the phase shift achieved by the low frequency transmission line for sound waves within the pass band of the low frequency transmission line is approximately the same as that achieved by the sound path through the passages 18 for frequencies above the pass band of the transmission line,

From FIGURES 1 and 2, the operation of the described microphone now should become apparent to those skilled in the art. The manner in which low frequencies are handled in order to obtain a unidirectional characteristic has previously been described. At intermediate frequencies, sound pressure from the low frequency opening will add vectorially with sound from the high frequency opening to produce a resultant sound pressure on the back of the diaphragm. This resultant sound pressure is equivalent to the sound pressure which would have been achieved at the rear of the diaphragm had its entrance been somewhere along the axis of the microphone between entrance 22A of the low frequency tube 22 and the entrance 17 of the high frequency passages 18. By selection of the parameters of the various openings, the effective opening to the back of the diaphragm can be made to assume an effective position along the axis of the microphone which is at a distance from the diaphragm inversely proportional to the frequency. This will produce an effective sound pressure for actuating the diaphragm which is virtually independent of the frequency while still maintaining a unidirectional characteristic over the frequency pass band.

At the extreme high frequency end of the pass band, the diameter of the microphone becomes comparable to the wave length. This will produce a more directional characteristic than the cardioid pattern produced at the lower frequencies.

FIGURE 3 is a vector diagram illustrating the relation of the prior art pressure gradient microphone having only a single opening to the back of the diaphragm 12. The force F is applied to the front of the diaphragm, and force F is applied to the back of the diaphragm. Force F is the resultant force available for actuating the diaphragm. The angle of the force F has been shown as 2KD where D is the acoustical distance from the back to the front of the diaphragm, and K is equal to where A is the wave length. The factor of 2 is employed where the microphone is of the cardioid type and the sound source is from the axial front of the microphone.

FIGURE 4 is a vector diagram of the same microphone plotted in FIGURE 3, but at the condition when the frequency of the sound is higher than that impinging under the conditions illustrated in FIGURE 3. A comparison of FIGURES 3 and 4, therefore, illustrates that the resultant force F for actuating the diaphragm is approximately proportional to the frequency if the wave length is large compared to the distance from the front to the back of the diaphragm. By such arrangement, where the force is proportional to frequency, the mechanical impedance of the generating element must be proportional to frequency in order to produce a velocity independent of frequency, and hence this means that the generating element must be mass controlled.

FIGURE 5 is a vector diagram of the present microphone where sound is arriving on the front axis of the microphone, and the effective distance between the front and the back of the microphone is a variable quantity. D is the acoustical distance from the front of the microphone to the low frequency opening 22A. At low frequencies the sound enters the back of the diaphragm at a much greater distance so that the angle 2KD is large enough to produce a high resultant force F In FIG- URE 5, D is large due to the low frequency sound since at a low frequency sound from the high frequency openings 17 is highly attenuated.

h FIGURE 6 is a vector diagram showing what happens at a higher frequency range, wherein D is the acoustical distance from the front of the diaphragm to the high frequency opening 17, and the force F is the force due to the high frequency sound passing through the high frequency passages to the rear of the diaphragm. Sound from the low frequency opening 22A in the microphone has been attenuated to a negligible quantity by the internal low pass filters as is apparent from an examination of FIGURES 1 and 2.

FIGURE 7 illustrates the vector diagram at a frequency in the mid range in which the force F due to the sound at the low frequency opening 22A and the force F due to the sound at the high frequency opening 17 are both of import, both having been attenuated by the filters in the low and high frequency passages. F, has an angle of 2KD whereas F has a greater angle of ZKD This produces a resultant force F +F which together with the force F produces a resultant effective force on the diaphragm F Comparing the representation in FIGURE 7 with the representations in FIGURES 5 and 6 shows that the resultant force on the diaphragm of a microphone constructed according to the present invention is substantially independent of the frequency.

In conventional pressure gradient microphones, a proximity effect is experienced which increases the output of the microphone for low frequencies if the microphone is in reasonably close proximity to the sound source. This is due to the magnitude of the sound pressure on the front of the diaphragm being appreciably higher than that exerted on the back of the diaphragm. In the previous vector diagrams, it was assumed that the sound source was at an appreciable distance from the microphone. In FIGURE 8, the vector diagram is that of a conventional pressure gradient microphone for a low frequency in reasonably close proximity to the sound source, and also when used at a distance. In both cases, it was assumed that the sound pressure exerted on the front of the diaphragm was the same.

Force F is the force applied to the front of the diaphragm in either case, F is the force on the back of the diaphragm when the microphone is at a distance of several wavelengths from the sound source thus producing a resultant F F is the force on the back of the diaphragm when the microphone is close to the sound source, which will give the resultant force of F which is considerably greater in magnitude than F This difference in the force explains the increase in the output of the microphone at bass or low frequencies. This effect, however, is not obtained in microphones embodying the present invention, since the resultant forces are comparable in magnitude.

FIGURE 9 shows the vector diagram of the forces on the diaphragm of a microphone comprising the present invention. F is the force applied to the front of the diaphragm. The resultant force F +F due to distant sounds is much greater than F '+F where the microphone is close to the sound source. The angle, however, is always large, so that the resultant force F for distant sources is very nearly the same as the resultant force F resulting from a sound source close to the microphone. From the foregoing it, therefore, should be apparent that in accordance with the present invention there has been produced a microphone which has a rugged resistance control generating action, one which is not susceptible to shock, and one where there is virtually no proximity effect.

Reference may now be had to FIGURES 10 and 11 which show certain details of a typical physical embodiment of the present invention. Those portions corresponding to the diagrammatic representation in FIGURE 1 have been given corresponding reference characters in FIGURES 10 and 11. The housing or casing 16 has an enlarged upper portion 31 terminating in an open end having internal threads 32 which are engaged by a three legged spider 33 and forming a sound port to the exterior of the housing. The spider has a central opening which is threaded to receive a thumb screw 34 to hold in position a wind screen 35. The three legged spider 33 engages a gasket or support 13 and the peripheral portion of the diaphragm 12. The diaphragm 12 has a depending an nular ring which carries the voice coil 14 into a gap in the magnetic structure 16A. The magnetic structure 16A includes iron portions and two or more permanent magnetic pieces $6. The high frequency opening 17 on opposite sides of the microphone, as is apparent from the front view of FIGURE 10, is protected by a suitable screen or grill 37. The magnetic circuit and structure 16 is mounted on a stack of insulating plates 38 bolted thereto by suitable bolts 39. The assembled stack of insulating plates 38 rests upon a gasket 41, and the entire assembly is retained in position by the three legged spider 33. A microphone transformer 42 is held in depending position from the insulating stack 38 by a plurality of bolts 43.

It will be noted that the lower portion of the casing 16' has a reduced diameter portion 44 to fit on a suitable microphone fixture. Further details disclosed in FIG- URES 10 and 11 are merely construction and design details for the physical embodiment, and do not require any specific description since they would be readily understood by anyone skilled in the design of microphone casings and the like, and are not necessary for a complete understanding of the principles of the present invention. Accordingly it is to be understood that the disclosure does not represent any limitation in the present invention which is applicable to microphone irrespective of the type of electro-mechanical translating means employed, since it is contemplated that such variations and other embodiments may be made as are commensurate with the spirit and scope of the invention in the accompanying claims.

I claim as my invention:

1. A pressure gradient microphone having directional properties within a frequency band comprising a housing having a sound port therein, an electroacoustical transducer having a diaphragm confronting the sound port on one side, said housing having first and second openings, the first opening being spaced from the sound port by a shorter distance than the second opening, said housing having means defining a first sound path between the first opening and the side of the diaphragm opposite the sound port and a second sound path between the second opening and said side of the diaphragm, said means including a chamber common to both sound paths and in communication with the diaphragm, an acoustical impedance operatively associated with the first sound path, means operatively associated with the second sound path for limiting the frequencies transmitted therethrough to a low frequency band, the first sound path transmitting a second band of frequencies above the low frequency band and including the higher frequencies of the low frequency band, the length of the first sound path shifting the phase of sound waves above the low frequency band approximately the same as the length of the second sound path shifts the phase of sound waves within the low frequency band, sound waves of a frequency within both the low and high frequency bands being transmitted to the diaphragm through both the first and the second sound paths.

2. A pressure gradient microphone comprising the elements of claim 1 wherein the housing is provided with a second chamber and a passage between the second chamber and the first chamber.

3. A pressure gradient microphone comprising the elements of claim 2 wherein the electroacoustical transducer comprises magnetic circuit means having an annular gap and a voice coil translatably disposed within the gap and mounted on the diaphragm.

4. A pressure gradient microphone comprising the elements of claim 1 wherein the means for limiting the sound waves transmitted through the second sound path to a low frequency band comprises a tube and a passage connected in cascade, the tube having a larger volume per unit length than the passage, and a plurality of acoustical resistance elements disposed at spaced intervals within the tube.

5. A pressure gradient microphone comprising the elements of claim 1 wherein the acoustical impedance of the first sound path comprises a felt member extending entirely across said first sound path.

6. A pressure gradient microphone comprising the elements of claim 4 in combination with an acoustical resist-- ance element disposed within the passage.

7. A pressure gradient microphone comprising a housing having a sound port therein, an electroacoustical transducer disposed within the housing having a dia-- phragm with one side confronting the sound port, said. transducer having magnetic circuit means with an annulargap and a voice coil translatably disposed within the gap and mounted on the diaphragm, said housing having firstv and second openings therein, the first opening being: spaced from the sound port by a shorter distance than the second opening, said housing having means defining a first chamber on the side of the diaphragm opposite the. sound port and a first passage communicating with the: first chamber and the first opening, said first passage and chamber forming a first sound path between the first opening and the side of the diaphragm opposite the sound port, an acoustical resistance element disposed in the first passage, said housing having means defining a tube and a second passage of smaller volume per unit length than the tube connected to one end of the tube, the other end of the tube being in communication with the second opening and the end of the second passage opposite the tube being in communication with the first chamber, the tube, second passage and the first chamber forming a second sound path between the second opening and the side of the diaphragm opposite the sound port, a plurality of acoustical resistance elements disposed at spaced intervals within the tube, said tube, second passage and resistance elements limiting sound Waves passing therethrough to a low frequency band, an acoustical resistance element disposed in the second passage, the first sound path transmitting a second band of frequencies above the low frequency band and including the higher frequencies of the low frequency band, the length of the first sound path shifting the phase of sound waves above the low frequency band approximately the same as the length of the second sound path shifts the phase of sound waves within the low frequency band, sound waves of a frequency within both the low and high frequency bands being transmitted to the diaphragm through both the first and second sound paths, said housing defining a second chamber and a third passage in communication with the second chamber at one end and the first chamber at the other end, and an acoustical resistance element in the third passage, said second chamber, third passage and resistance element forming a terminating impedance for the second sound path.

8. A pressure gradient microphone comprising the elements of claim 1 wherein the housing is elongated and the sound port is disposed at one end thereof, said housing being provided with a third opening at the same distance from the sound port as the first opening and on the side of the housing opposite the first opening, said housing having means defining a third sound path between the third opening and the side of the diaphragm opposite the sound port, said third sound path being approximately the same length as the first sound path and including the chamber in common with the first and second sound paths, and a second acoustical impedance operatively associated with the third sound path.

9. A directional microphone for transmitting a predetermined range of frequencies which are audible by the human ear, comprising a diaphragm, means defining at least one elongated cavity coupled to one side of said diaphragm and which has a length lying between the shortest Wavelength and the longest wavelength of said range of frequencies, said means defining at least one elongated cavity including apertured portions disposed at points having different distances from said diaphragm to expose said one side of the diaphragm to the sound field surrounding the microphone, and mounting means carrying said diaphragm and exposing the other side of said diaphragm to the sound field surrounding the microphone.

10. A directional microphone as set forth in claim 9, in which said means comprise a tube of uniform crosssection.

11. A directional microphone as set forth in claim 9, in which said means comprise a tube having a free end and increasing in cross-section towards said free end.

12. A directional microphone as set forth in claim 9, which comprises means defining an air chamber between the diaphragm and said cavity.

13. A directional microphone as set forth in claim 9, which comprises means defining an air chamber connected to the diaphragm in parallel with said cavity.

14. A directional microphone as set forth in claim 9, in which said means defining said cavity are formed with at least one slot.

15. A directional microphone as set forth in claim 9, in which the front side of the diaphragm is directly exposed to the sound field and which comprises a rigid plate defining with the rear side of said diaphragm an air chamber between said diaphragm and said cavity which constitutes an acoustic impedance.

16. A directional microphone as set forth in claim 15, which comprises a magnetic transmitter.

17. A directional microphone as set forth in claim 15, which comprises means defining at least one additional acoustic impedance coupled to said air chamber.

18. A directional microphone as set forth in claim 9, in which said means comprise a bundle of tubes of stepped length.

References Cited in the file of this patent UNITED STATES PATENTS 2,216,961 Sanial Oct. 8, 1940 2,252,846 Giannini et al Aug. 19, 1941 2,401,328 Black June 4, 1946 2,549,963 De Boer et a1 Apr. 24, 1951 2,848,561 Gorike Aug. 19, 1958 OTHER REFERENCES Serial No. 409,712, Gorike (A.P.C.), published May 18, 1943. 

1. A PRESSURE GRADIENT MICROPHONE HAVING DIRECTIONAL PROPERTIES WITHIN A FREQUENCY BAND COMPRISING A HOUSING HAVING A SOUND PORT THEREIN, AN ELECTROACOUSTICAL TRANSDUCER HAVING A DIAPHRAGM CONFRONTING THE SOUND PORT ON ONE SIDE, SAID HOUSING HAVING FIRST AND SECOND OPENINGS, THE FIRST OPENING BEING SPACED FROM THE SOUND PORT BY A SHORTER DISTANCE THAN THE SECOND OPENING, SAID HOUSING HAVING MEANS DEFINING A FIRST SOUND PATH BETWEEN THE FIRST OPENING AND THE SIDE OF THE DIAPHRAGM OPPOSITE THE SOUND PORT AND A SECOND SOUND PATH BETWEEN THE SECOND OPENING AND SAID SIDE OF THE DIAPHRAGM, SAID MEANS INCLUDING A CHAMBER COMMON TO BOTH SOUND PATHS AND IN COMMUNICATION WITH THE DIAPHRAGM, AN ACOUSTICAL IMPEDANCE OPERATIVELY ASSOCIATED WITH THE FIRST SOUND PATH, MEANS OPERATIVELY ASSOCIATED WITH THE SECOND SOUND PATH FOR LIMITING THE FREQUENCIES TRANSMITTED THERETHROUGH TO A LOW FREQUENCY BAND, THE FIRST SOUND PATH TRANSMITTING A SECOND BAND OF FREQUENCIES ABOVE THE LOW FREQUENCY BAND AND INCLUDING THE HIGHER FREQUENCIES OF THE LOW FREQUENCY BAND, THE LENGTH OF THE FIRST SOUND PATH SHIFTING THE PHASE OF SOUND WAVES ABOVE THE LOW FREQUENCY BAND APPROXIMATELY THE SAME AS THE LENGTH OF THE SECOND SOUND PATH SHIFTS THE PHASE OF SOUND WAVES WITHIN THE LOW FREQUENCY BAND, SOUND WAVES OF A FREQUENCY WITHIN BOTH THE LOW AND HIGH FREQUENCY BANDS BEING TRANSMITTED TO THE DIAPHRAGM THROUGH BOTH THE FIRST AND THE SECOND SOUND PATHS. 